A milestone achievement in the bone cement surgery was Charnley's development of polymethyl methacrylate (PMMA) bone cement used for artificial joint fixation in 1959. Almost forty years later, PMMA remains the standard material for anchoring total joint implants to the skeleton. During this time, there have been numerous improvements in prosthesis design and implantation techniques to improve the clinical, restorative and rehabilitative aspects of the total hip arthroplasties. The success rate of cemented hip arthroplasties at 10 years exceeds 90% in patients aged 60 years or more when proper cementing techniques are used. However, despite the improvements, PMMA has several recognized shortcomings as a structural material. Aseptic loosening remains the major long-term problem with total joint replacement.
In a bone cement system, there are three different materials (bone, cement, and implant) and two interfaces (bone and bone cement, bone cement and implant). The properties at the interfaces are mismatched because the cement is much weaker than the bone and the implant. Fatigue and fracture of cement were implicated in the failure of these devices. Loosened prostheses, as defined by Harris et al. generally require revision surgery. The total cost of a primary total hip arthroplasty is substantial. The cost of revision surgery may be even greater. The life expectancy of a revised prosthesis is considerably lower than that of a primary prosthesis; furthermore, the trauma and pain associated with the primary prosthesis failure, and the revision operation, are taxing on the patient. The reduction of the failure rate in joint arthroplasty, therefore, is a primary goal of biomechanics and biomaterials research.
Classic bone cements are bi-component materials, which are composed of MMA, PMMA, initiators and filler. The solid material forms a plastic paste upon mixing with the liquid phase, usually under vacuum, with a specially designed apparatus. This viscous paste is then transferred into the human body between the prosthesis and bone using a cement delivery system. During this time, the paste solidifies, increasing its mechanical strength progressively up to saturation.
Ever since the introduction of surgical bone cements, there have been many efforts to improve their mechanical properties. Steps to improve the strengths of bone cement have been categorized into various distinctly divergent paths. Addition of reinforcing fiber, designing of new mixing and delivery methods, modifying the powder or liquid components, bioactive cements, and even cementless technology are major fields of focus. Even though there have been some remarkable improvements in bone cement technologies for clinical application, however, not all efforts to produce superior quality bone cement for total joint replacement have so far been successful.
Efforts to improve bone cement properties have been extensive. These efforts largely have involved modification of monomer components and cross linkers. Pascual et al. replaced up to 20%, of the monomer methylmethacrylate (MMA) with the same amount of ethoxytriethylene glycol monomethacrylate (TEG). They found that the addition of this new monomer decreased noticeably the maximum temperature and increased both setting and working times. Mechanical testing revealed that the introduction of TEG gave rise to a less fragile bone cement by increasing slightly the total deformation without any change in the rest of the tensile parameters.
Crosslinking agents, usually bifunctional dimethacrylates, were used to try to improve the mechanical properties of the acrylic bone cements. At low concentrations of crosslinking agents, the mechanical properties were superior but steadily decreased with increasing concentration. Poly(ethyleneglycol dimethacrylate), EGDMA(400), even when used at very low concentrations, produced a steady improvement in the mechanical properties and could be used in cement formulations with a view to reducing creep and improving mechanical properties.
The same strategy has been applied to PEMA bone cement. Incorporation of triethylene glycol dimethacrylate produced an increase in the tensile strength and modulus with a decrease in the strain at maximum stress. However, polyethylene glycol dimethacrylate (n=400) did not improve the mechanical properties appreciably.
4-methacryloyloxyethyl trimellitate anhydride (4-META) has been added to monomer component as an adhesion promoting agent. Implantation of the 4-META cement in animals demonstrated that these cements did not disturb bone ingrowth and the new bone was able to contact the cement directly. A methacrylic monomer derived from salicylic acid, 5-hydroxy-2-methacrylamidobenzoic acid (5-HMA), was incorporated with 2-hydroxyethyl methacrylate, (HEMA), in different proportions to the liquid phase of acrylic bone cement formulations. 5-HMA monomer shows the ability to form molecular complexes with calcium atoms in order to improve osteointegration in the application of bone cement formulations. Lower peak temperature values were observed when 5-HMA was incorporated with respect to PMMA bone cement.
Hydroxypropyl methacrylate (HPMA) was also one of the candidates to modify bone cements. However, the adhesion properties were unsatisfactory. The problem was solved by increasing the monomer to polymer ratio from 1:2 to 1:1.86.
Radiopaque iodine-containing methacrylate, 2,5-diiodo-8-quinolyl methacrylate and 5,7-diiodo-8-quinolyl methacrylate have been used in the preparation of acrylic radiopaque cements.] The addition of 5 wt % of the iodine-containing methacrylate provided a significant increase in the tensile strength, fracture toughness and ductility, with respect to the barium sulfate-containing cement, since no organic/inorganic interface exist in this system.
Polybutyl methacrylate (PBMA) and polyethyl methacrylate (PEMA) have been used to replace PMMA. Butyl methacrylate monomer was believed to be slightly less toxic than methyl methacrylate monomer. The surface appearance of the broken cement from the two materials differed significantly, showing a series of elevations resembling tightly packed spheres in the case of PMMA, but a smooth surface with only occasional smooth elevations in the case of PEMBMA. PBMA and PEMA modified bone cement also show less bone necrosis and a thinner fibrous tissue layer adjacent to the cement when it is cured intraosseously.
Bone cement formulated from polybutyl methacrylate in a methacrylate matrix (PBMMA) can reduce the modulus of the materials. However, it has much greater long-term subsidence of the implant system.
Although polyethyl methacrylate (PEMA) offers a promising alternative to PMMA due to its high ductility, low toxicity and low exotherm, the fatigue test revealed that specimens made of PEMA was inferior to the that of PMMA in term of the number of cyclic loadings to failure. If HA is fabricated and mixed with PEMA, it can potentially result in an increase in fracture toughness, fatigue crack propagation resistance, and creep resistance, without a decrease in adhesive strength, with decrease in toxicity of acrylic cements, however, the cycles to failure were decreased. When HA particles are treated with a silane-coupling agent, the fatigue strength is enhanced as well.
Traditionally, poly(methyl methacrylate-co-styrene) was synthesized by suspension polymerization. The powder is then mixed with barium sulfate and benzoyl peroxide. The proper formulation of the powder package is still under observation. Synthesis of copolymers of methyl methacrylate-styrene with suitable compounds, particle size distribution, molecular weight and molecular weight distribution for bone cement application have been discussed by Cordovi et al.
Self-reinforced composite poly(methyl methacrylate) (SRC-PMMA) was developed by Wright et al. to use as a pre-coat for hip prostheses or other stemmed prostheses. This material has a similar chemical composition to bone cement, with the matrix and reinforcing fibers both fabricated from PMMA.
The elastomeric copolymer acrylonitrile-butadiene-styrene (ABS) was found to be an excellent material to enhance the mechanical properties of acrylic bone cement. Although strength and stiffness decreased with an increasing second phase volume fraction, ductility and toughness both increased. The crack propagation became stable for specimens containing over a 5% volume fraction of the second phase. The fracture toughness increased up to 60% when the amount of ABS reached 20%. Fatigue crack propagation rate decreased by about 2 orders of magnitude. Size of PMMA Beads
PMMA beads for bone cement application generally are made by emulsion polymerization. Ginebra et al. concluded that the use of relatively larger diameter PMMA beads improves the characteristic parameters of the curing process, without detrimental effects on the mechanical properties of the cured cement. Pascual et al. revealed that changing the size distribution of the PMMA beads significantly changes the curing parameters (peak temperature and setting time) of the cement formulations in comparison with the classical behavior of the commercial systems, CMW and ROSTAL, without any noticeable loss in the mechanical properties, such as tensile strength, elastic moduli, compressive strength and plastic strain.
Solutions of PMMA powder predissolved in MMA have been developed as an alternative to current powder/liquid bone cements. They utilized the same addition polymerization chemistry as commercial cements, but in mixing and delivering via a closed system, porosity is eliminated and the dependence of material properties on the surgical technique is decreased. The system is composed of two separate packages with two solutions of constant polymer-to-monomer ratio, but one having BPO initiator and the other having NNDMPT activator. The mechanical properties could be superior to traditional bone cements. We found that the shelf life is not promising for this approach.
For example, the potential advantage of increasing the mechanical strength of bone cement by adding fibers is offset by the increase in the viscosity of the cement. Cementless technology is inapplicable to aged people since bone ingrowth can be difficult to achieve. Even the efforts to reduce the porosity of surgical bone cement were not effectively linked to the long-term outcomes of total hip arthroplasty. On the other hand, there is evidence that bone cement fracture does lead to a certain percentage of prosthesis failure. Therefore, in theory, an improvement in the resistance of the cement material to fracture might also be pivotal to perfecting the overall performance of cemented prosthesis. Thus, there exists a need for a bone cement with overall superior handling and mechanical properties.
The cement typically is provided in two components, powder and liquid monomer. The physician mixes the two shortly before use to form a pourable liquid, which is loaded into a syringe made for the purpose. The liquid rapidly thickens into a viscous paste, requiring considerable force for ejection from the syringe. The syringe is put into a hand-held injector, whereby the viscous paste can be forced out of the syringe into the bone as detailed in U.S. Pat. No. 4,405,249.
During the operation of mixing the cement components and filling the syringe, bubbles of air are inevitably entrained in the liquid; when the liquid thickens, the bubbles cannot escape from the paste. The bubbles of air are expressed with the cement into the bone; and when the cement hardens, the bubbles leave voids in the solidified cement. Thus, there exists a need for a cement delivery system that facilitates rapid mixing and limits bubble entrainment.